The present invention relates to an internal bubble cooling (IBC) air control for a plastic blown film apparatus.
When blown film is extruded, it typically is in the form of a continuous, vertically oriented tube. The tube, which is in a molten state as it exits a die, expands in diameter as it is pulled continuously upward. The diameter stabilizes to a more or less constant value when the tube cools sufficiently to solidify a short distance from the die at what is called the frost line. Air cooling systems such as external air rings and IBC systems within the tube are provided close to the exit of the die to ensure that the tube cools quickly enough to remain stable.
The tube usually passes through a bubble cage, which minimizes unwanted tube motion and also determines the final tube size if the cage is allowed to contact the tube while the tube is still molten. After solidifying, the tube passes through a flattening device, known as a collapsing frame, to convert the inflated tube into a flattened out film with no air inside. This film is pressed together by motorized nip rolls that continually draw the film upward and away from the extrusion process to form what is call xe2x80x9clayflat.xe2x80x9d The die and nip roll act as seals, which in steady state, form a trapped, column of air with constant volume inside the tube.
Film processors employing IBC systems realize production rate gains on the order of 20% to 50%. In known IBC systems, such as that described in U.S. Pat. No. 4,243,363, air passages are provided through the die to allow for significant air flow into and out of the tube. Air supply and exhaust systems act under the supervision of a control system in response to measured tube size. The control system adjusts the flow of air to be in balance so that a constant, desired tube size is maintained.
For IBC systems to remain stable, there cannot be a significant closed loop lag time between the time when an air flow change first occurs and when the new tube size actually gets sensed by the controller. Excessive total lag time causes a tube to oscillate in size. Typical oscillation periods induced by IBC systems are generally 4 to 6 seconds in duration. This implies that the closed loop lag time must remain less than about 1 second to 1.5 seconds or else the lag time will be greater than 90 degrees out of phase and oscillation will result. Present art IBC systems have a hardware sensor response time and actuation of corrective air flow lag time of about xc2xd to 1 second. Total closed loop lag time includes this hardware lag time and an additional process related sensing lag time caused by size changes taking place at a point prior to where sensing occurs.
Bubble instability prevents film processors from using IBC systems to achieve higher production rates when extruding many of the newer high performance materials. This instability is caused by a process related sensing lag time that is great enough to force the IBC system into oscillation. Sensing lag time is the time it takes for the molten region of the bubble, which reacts in size to the influence of IBC air flow changes, to move along the process until it has solidified into a final dimension that can be accurately measured at or just after the frost line. Traditionally, older resins such as Low Density Polyethylene (LDPE) react in size to air flow changes very close to or at the frost line, thus providing for minimal sensing time lag time and making it easy to control bubble stability. In the early 1980""s, however, Linear Low Density Polyethylene (LLDPE) became commercially viable. LLDPE reacts just prior to reaching the frost line causing a slightly longer sensing time lag of xc2xd to 1 second. Processors found LLDPE more difficult to control, but by blending in small amounts of LDPE and/or by lowering the IBC size sensors to the frost line or slightly below the frost line, bubble stability could be maintained. Lowering the sensors this far, however, has a disadvantage in that the measured size is no longer accurate. This accuracy problem has been partially addressed by control systems providing for easy re-calibration of measured tube size. More recently, new materials such as metallocenes have further lowered the reaction point, making them difficult or impossible to control with IBC systems. It would be advantageous to sense directly at a reaction point that is well below the frost line without adverse effects on measured size.
An additional problem arises due to the sensitivity of sensor positioning in that the frost line does not stay in one place over time. As material and ambient conditions vary during production, the location of the frost line can change by several inches. This movement causes the processor to constantly monitor and adjust sensor positioning to track with the frost line. Presently, sensor adjustments are made manually by the operator, usually in response to tube oscillation that suddenly appears or actual tube size changes that occur due to degraded tube size calibrations. It would be advantageous to automatically reposition IBC sensors relative to the frost line to maintain sensing lag time constant, thus preventing the onset of tube oscillations. Automatic positioning would also serve to minimize the need for tube size re-calibration although bubble shape effects that can accompany changes in the location of the frost line might still warrant re-calibration, but significantly less often.
Another problem relates to a well documented characteristic that tubes naturally vary in size over short periods of time, independent of any IBC volumetrically related instability, just as processes not using IBC systems do. Experimentation has revealed that with materials in use today, tube size naturally changes in a periodic manner with a frequency of about 1 to 2 Hertz. Tube size changes by as much as ⅓ to xc2xd inch of layflat for processes with light or no contact with the bubble cage and from {fraction (1/10)} to ⅓ inch for processes that use the bubble cage to squeeze in just below the frost line and size the bubble. It is a disadvantage to squeeze the tube since marks and scratches routinely result from contact points with the bubble cage. Without squeezing with a cage, IBC control systems must have a total system response time (sensing lag time+sensing response time+actuator response time) of about 0.1 seconds (10 hertz) or better to control these natural fluctuations. Presently, sensing lag time, response time, and actuator response time individually are each too great to allow for control of natural tube size changes so each must be addressed. It would be beneficial if total system lag time and accuracy could be brought to a level where higher frequency natural bubble instabilities could be controlled by IBC control systems without reducing film quality due to scratches.
IBC control systems employ mechanical, optical, and acoustic sensors for monitoring tube size. Mechanical sensors cause marks on the resulting film and optical sensors tend to get dirty and unreliable in the typical blown film plant environment making them unsuitable for many applications in blown film production. Acoustic sensors are preferred because they provide non-contact sensing and are very reliable in a plant environment. Such systems, however, do have slow sampling rate and problems with sensor interference when more than one acoustic sensor is placed into service around a tube. Acoustic sensors operate by sending out a conical ultrasonic sound pulse and measuring the time it takes for the pulse to bounce off a target, such as the tube, and return back to the sensor which sent the pulse. Distance is then calculated by multiplying the time of flight by the speed of sound in the ambient air that the pulse just traveled through. Blown film bubbles tend to flutter and move around, causing the sound pulses to bounce in many different directions. If the pulse passes by a sensor other than the one that sent it, interference can and usually does occur. Additionally, an originating sensor may not receive the return signal, resulting in a missed target response. Yet another problem is that intervening objects such as operating personnel servicing the system can prevent the acoustic sensor from detecting the bubble as a target, and instead such personnel become the target. These errors lead to incorrect reaction by the control system and thus cause instability in the tube size. Present art systems employ various techniques to minimize these problems at the expense of response time.
A common method and the least expensive overall acoustical approach is to use only one sensor as described in U.S. Pat. No. 5,104,593. This approach suffers the most from inaccuracy due to tube motion. The swaying motions and the flutter of the tube common to the blown film process is perceived by the sensor to be a change in size with corresponding degradation in performance. To combat this inaccuracy and achieve good size control, single sensor systems typically require the use of a bubble cage to surround the tube and forcefully determine tube size, thereby causing scratches and marks in the finished film. Interference with other sensors is not an issue, and this approach allows for sampling rates of 25 to 30 times per second, but a dual stage software filtering system is required to prevent misidentifying noise or bubble sway as an actual change in tube size thus allowing it to track only relatively gradual changes in tube size. The first stage of the filter requires an average of 8.5 samples to effect a change in output yielding an approximate ⅓ second response time. The second stage further limits response time.
Another common method, as described in U.S. Pat. No. 4,377,540, is to use more than one acoustic sensor by alternately operating each sensor one at a time in sequence and wait long enough between samples to prevent interference. In this approach multiple sensors sample tube size preferably from diametrically opposed positions, thus canceling the effects of tube sway. Due to the time delays present and lower operating frequencies, however, these systems allow for only 10 samples to be taken per second. True diameter measurements without influences from swaying require at least 2 samples limiting this approach to ⅕ second sensing response which is then further limited by filtering elements necessary for outside noise immunity.
Yet another approach uses multiple sensors operating in a free run mode with sampling rates of 40 to 50 times per second without regard for alternating sensor operation. Here, sensors are placed so that stray signals typically bounce away from one another. Interference can still occur, however, so a special rate filter is employed to eliminate the effects of inter-sensor interference and missed targets. Experiments have determined that this approach has a typical sensor response time on the order of ⅕ to xe2x85x9 second.
None of the present systems can tolerate the accidental placement of intervening objects in front of acoustic sensors. Typically, objects placed in front of sensors lead to significant bubble instability sufficient to force the extrusion line to shut itself down.
Significant limitations also exist with actuators that adjust air flow. Most IBC air flow actuators are butterfly style valves. These valves suffer from inaccuracy due to linkage backlash and are either motorized or respond to the position of a spring loaded air cylinder. Other actuators are of the bladder valve design, which has no backlash, and operates by inflating or deflating a series of bladders contained inside the air system piping to change the resulting air flow restriction. Yet another design uses a spring loaded air cylinder to position a linear slide valve that also has no backlash, but suffers from problems with positioning errors due to drag in the valve and air cylinder. Experiments have revealed that motorized valves have reaction times of about xc2xd to 1 second, while spring loaded air cylinders and bladders use pneumatic systems that move air to pressurize or depressurize the actuator with total reaction time of ⅓ to xc2xe of a second. Unfortunately, actuators generally in use today in blown film systems do not have the reaction time or accuracy required for controlling natural high frequency bubble instability.
The present invention includes an internal bubble cooling (IBC) and control system using acoustic sensors that measure tube size resulting from the blown film extrusion apparatus. The IBC control system provides for size sensors located above the frost line where the size of the bubble is stable, to maintain tight calibration of actual tube size. Separate control sensors are adjustable in position at a vertical location below that of the size sensors. These control sensors are preferably located at or below a point just above the frost line, and may be well below the frost line, so that in production these control sensors can be positioned at the point of maximum bubble size reaction to internal air flow changes to compensate for high speed size fluctuations.
Size and control sensors initially are calibrated by having operating personnel inputting actual manually measured size into the system and applying this size data independently to each sensor to establish a separate respective calibration value. Size sensor calibrations remain fixed until a next operator calibration. These size sensors are then used in an integrating mode to constantly re-calibrate each separate control sensor, thus allowing them to be located wherever necessary just above or below the frost line to control the process.
The initial calibrations for control sensors are stored separately and are compared to the integrated re-calibration ongoing for each control sensor. As the position of the frost line naturally changes over time, the control sensor location is automatically adjusted, usually by means of positioning the sizing cage to which they are attached. Position adjustments are made until the integrated re-calibration again matches the initial calibration for the control sensors.
Signals representing a position of the bubble are then provided to control logic in a controller to cause more or less cooling air to flow onto the bubble.
The present invention also includes a sensing method that requires no time averaging of signals to eliminate bad readings and allows for full speed operation of the sensors. Preferably, more than one sensor is used for sizing and more than one is used for controlling. Use of multiple sensors provides a redundancy that allows for rapid filtering by analyzing each sensor""s response for false readings. Statistically, there will be at least one sensor that detects the bubble close to where it has been within a tolerance of preferably 1 to 2 inches from previous measurements. All sensors are compared to one another, and any sensor that falls outside a specified tolerance band are ignored. Further, if a majority of the responses from a given sensor are bad, that sensor is automatically taken out of service without shutting down the process. A warning, such as a warning light, can be turned on (or a normally xe2x80x9conxe2x80x9d light turned off) separately for each sensor to inform the operator that a sensor is being automatically ignored; if a sensor is being ignored, the warning light remains on permanently until the sensor begins to again provide a majority of good responses.
The present invention further includes a control system which synchronizes and simultaneously fires all acoustic sensors, and then waits a delay time, such as 3 to 16 milliseconds (depending on sensor arrangement), that is long enough for any stray signals to bounce harmlessly away without causing inter-sensor interference before repeating the sequence. By desirably positioning size and control sensors each as pairs of diametrically opposed sensors, true bubble size measurements can be made within a single multiple-sensor cycle without the unwanted effects of bubble sway and without interference problems. Combining synchronized rapid firing with redundancy filtering allows for simultaneous, reliable and accurate measurement of the tube for control and sizing between 60 and 300, and preferably 100, times per second, with no need for further filtering.
Yet another feature of the present invention relates to a linear valve which both precisely meters the amount of air flow and actuates at a very high speed. The linear valve operates by positioning a piston inside of a double pipe arranged with longitudinal slots that, when partially uncovered, control the amount of air flowing from the innermost to the outermost pipe. According to the invention, a linear servo motor is employed (rather than an air cylinder) for piston positioning together with a vertical orientation, and the piston and wall around the piston are designed for minimal friction to yield fast and precise metering of air flow.
Other features and advantages will become apparent from the following detailed description, drawings, and claims.